1. Field of the Invention
This invention relates to mobile communications and, more particularly, to a system and method of group calling in mobile communications.
2. Discussion of Related Art
All modern mobile communication systems have a hierarchical arrangement, in which a geographical “coverage area” is partitioned into a number of smaller geographical areas called “cells.” Referring to FIG. 1, each cell is preferably served by a Base Transceiver Station (“BTS”) 102a. Several BTS 102b–n are aggregated via fixed links 104a–n into a Base Station Controller (“BSC”) 106a. The BTSs and BSC are sometimes collectively referred to as the Base Station Subsystem (“BS”) 107. Several BSCs 106b–n may be aggregated into a Mobile Switching Center (“MSC”) 110 via fixed links 108a–n. 
MSC 110 acts as a local switching exchange (with additional features to handle mobility management requirements, discussed below) and communicates with the phone network (“PSTN”) 120 through trunk groups. Under U.S. mobile networks, there is a concept of a home MSC and a Serving MSC. The home MSC is the MSC corresponding to the exchange associated with a Mobile Station (“MS”); this association is based on the phone number, e.g., area code, of the MS. (The home MSC is responsible for the HLR discussed below.) The Serving MSC, on the other hand, is the exchange used to connect the MS call to the PSTN (as the subscriber roams in the area covered by the service provider, different MSCs perform the function of the Serving MSC). Consequently, sometimes the home MSC and the Serving MSC are the same entity, but other times they are not (e.g., when the MS is roaming). Typically, a Visiting Location Register (“VLR”) 116 is co-located with the MSC 110 and a logically singular HLR is used in the mobile network. As will be explained below, the HLR and VLR are used for storing many types of subscriber information and profiles.
Briefly, one or more radio channels 112 are associated with the entire coverage area. The radio channels are partitioned into groups of channels allocated to individual cells. The channels are used to carry signaling information to establish call connections and the like, and to carry voice or data information once a call connection is established.
At a relatively high level of abstraction, mobile network signaling involves at least two main aspects. One aspect involves the signaling between an MS and the rest of the network. With 2G (“2G” is the industry term used for “second generation”) and later technology, this signaling concerns access methods used by the MS (e.g., time-division multiple access, or TDMA; code-division multiple access, or CDMA), assignment of radio channels, authentication, etc. A second aspect involves the signaling among the various entities in the mobile network, such as the signaling among MSCs, VLRs, HLRs, etc. This second part is sometimes referred to as the Mobile Application Part (“MAP”) especially when used in the context of Signaling System No. 7 (“SS7”).
The various forms of signaling (as well as the data and voice communication) are transmitted and received in accordance with various standards. For example, the Electronics Industries Association (“EIA”) and Telecommunications Industry Association (“TIA”) help define many U.S. standards, such as IS-41, which is a MAP standard. Analogously, the CCITT and ITU help define international standards, such as GSM-MAP, which is an international MAP standard. Information about these standards is well known and may be found from the relevant organizing bodies as well as in the literature, see, e.g., Bosse, SIGNALING IN TELECOMMUNICATIONS NETWORKS (Wiley 1998).
To deliver a call from an MS 114, a user dials the number and presses “send” on a cell phone or other MS. The MS 114 sends the dialed number indicating the service requested to the MSC 110 via the BS 107. The MSC 110 checks with an associated VLR 116 (more below) to determine if the MS 114 is allowed the requested service. The Serving MSC routes the call to the local exchange of the dialed user on the PSTN 120. The local exchange alerts the called user terminal, and an answer back signal is routed back to the MS 114 through the serving MSC 110 which then completes the speech path to the MS. Once the setup is completed the call may proceed.
To deliver a call to a MS 114, (assuming that the call originates from the PSTN 120) the PSTN user dials the MS's associated phone number. At least according to U.S. standards, the PSTN 120 routes the call to the MS's home MSC (which may or may not be the one serving the MS). The MSC then interrogates the HLR 118 to determine which MSC is currently serving the MS. This also acts to inform the serving MSC that a call is forthcoming. The home MSC then routes the call to the Serving MSC. The serving MSC pages the MS via the appropriate BS. The MS responds and the appropriate signaling links are setup.
During a call, the BS 107 and MS 114 may cooperate to change channels or BTSs 102, if needed, for example, because of signal conditions. These changes are known as “handoffs,” and they involve their own types of known messages and signaling.
One aspect of MAP involves “mobility management.” Briefly, different BSs and MSCs may be needed and used to serve an MS, as the MS 114 roams to different locations. Mobility management ensures that the Serving MSC has the subscriber profile and other information the MSC needs to service (and bill) calls correctly. To this end, MSCs use a Visiting Location Register (“VLR”) 116 and a Home Location Register (“HLR ”) 118. The HLR is used to store and retrieve the mobile identification number (“MIN”), the electronic serial number (“ESN”), MS status, and the MS service profile, among other things. The VLR stores similar information in addition to storing an MSC identification that identifies the (Home) MSC. In addition, under appropriate MAP protocols, location update procedures (or registration notifications) are performed so that the home MSC of a mobile subscriber knows the location of its users. These procedures are used when a MS roams from one location to another or when a MS is powered on and registers itself to access the network. For example a location update procedure may proceed with the MS 114 sending a location update request to the VLR 116 via the BS 107 and MSC 110. The VLR 116 sends a location update message to the HLR 118 serving the MS 114, and the subscriber profile is downloaded from the HLR 118 to the VLR 116. The MS 114 is sent an acknowledgement of a successful location update. The HLR 118 requests the VLR (if any) that previously held profile data to delete the data related to the relocated MS 114.
FIG. 2 shows in more detail the signaling and user traffic interfaces between a BS 107 and an MSC 110 in a CDMA mobile network. The BS 107 communicates signaling information using the A1 interface. The A2 interface carries the user traffic (e.g., voice signals) between the switch component 204 of the MSC and the BS 107. The A5 interface is used to provide a path for user traffic for circuit-switched data calls (as opposed to voice calls) between the source BS and the MSC.
Moreover, subscribers are demanding newer services, e.g., “data calls” to the Internet. For some of these services MSCs are not cost effective because they were primarily designed for voice calls. Integration of new services into the MSC is complicated or infeasible because of the proprietary and closed designs used by many MSC software architectures. That is, the software logic necessary to provide the services is not easy to add to the MSC 110. Often, a switch adjunct is used to provide such services. For example, an Inter-Working Function (“IWF”) is an adjunct to route a data call to the Internet. Either approach—integrating functionality into the MSC or adding a trunk-side adjunct—involves the MSC in the delivery of service. Since the new service is expected to spur demand, integrating new services via MSC design changes or through trunk-side adjuncts is likely to exacerbate network congestion at the MSC and require costly MSC resources.
With respect to the Internet, multicast communication refers to the transmission of identical data packets to selected, multiple destinations on an Internet protocol network. (In contrast, broadcast communication refers to the indiscriminate transmission of data packets to all destinations, and unicast communication refers to the transmission of data packets to a single destination.)
Each participant in a multicast receives information transmitted by any other participant in the multicast. Users connected to the network who are not participants in a particular multicast do not receive the information transmitted by the participants of the multicast. In this way, the multicast communication uses only the network components (e.g., switches and trunks) actually needed for the multicast transmission.
In multicast processing, when a potential participant (“host”) is directed to join a particular IP multicast group, the host sends a “request to join” message to the nearest multicast-capable router to request to join the multicast group and receive information sent to this group. For example, a host A sends a message to join multicast group Y, and a host B sends a message to join multicast group X. A router R propagates the request up to the multicast source if the data path is not already in place.
Upon receiving an IP packet for group X, for example, the router R maps an IP multicast group address into an Ethernet multicast address, and sends the resultant Ethernet packet to the appropriate switch or switches.
According to the current Internet Group Management Protocol (“IGMP”) a host's membership in a multicast group expires when the router does not receive a periodic membership report from the host.
With respect to interaction among MSs, a Nextel service (known as Nextel Direct 10 Connect®, using Specialized Mobile Radio technology, and described at http://www.nextel.com/phone—services/directconnect.shtml) having two versions has been proposed for special connection calls among MSs. Both versions of the special connection calls require that all members be located in the same area served by one BSC. In the first version, a one to one conversation is allowed between two mobile telephone subscribers, e.g., A and B. When A wishes to have special connection communication with B, A enters B's private identification number, holds down a push to talk (“PTT”) button, waits for an audible alert signifying that B is ready to receive, and starts speaking. To listen, A releases the PTT button. If B wishes to speak, B holds down the PTT button and waits for an audible confirmation that A is ready to receive. The service allows a subscriber to choose private identification numbers from scrollable lists displayed on mobile telephone handsets or to search a list of pre-stored names of subscribers.
In the second version, conversations are allowed among members of a pre-defined group of subscribers, known as a Talkgroup, which is identified by a number. The mobile telephone handset allows Talkgroup numbers to be searched through the control surface of the handset. In order to place a group call, the initiating subscriber, e.g., A, locates a Talkgroup number in the handset, holds down the PTT button, and, upon receiving an audible confirmation such as a chirp, can start speaking. All of the other Talkgroup members on the group call can only listen while A is holding down the PTT button. If A releases the PTT button, another member on the group call may hold down the PTT button, acquire control signaled by the audible confirmation, and start speaking.